1. Field of the Invention
The present invention relates to an optical fiber amplifier, and more particularly, to an optical fiber amplifier with improved small signal gain and having a low luminous intensity by connecting an optical coupler in the form of a loop mirror.
2. Description of the Related Art
In a general repeater for long distance communications, a conventional optical communications repetition is performed in such a manner that a weakened light signal is transformed into an electrical signal, the electrical signal is amplified, and then this amplified electrical signal is transformed back into a light signal. Such a repeating method excessively increases the size of a repeating amplifier system and varies with transmission speed. An optical fiber amplifier for amplifying an optical signal is required as a repeater to overcome the above problem and effectively amplify an optical signal.
An erbium doped fiber amplifier (EDFA) as such a fiber amplification repeater is attracting attention as a next-generation optical repeater for optical communications. The EDFA periodically amplifies an optical signal to prevent attenuation of the optical signal due to long distance transmission, when much data is transmitted over a long distance through an optical fiber.
FIG. 1 is a block diagram showing the configuration of a normal single forward EDFA comprising an erbium doped fiber (EDF) 130 being an optical signal amplifying medium, a pump laser diode (LD) 120, a wavelength division multiplexer (WDM) 110, and isolators 100 and 140. Here, the pump LD 120 functions as a light source for exciting erbium ions in a ground state in the EDF 130. The WDM 110 couples signal light and pump light of different wavelengths to an optical fiber. The isolators 100 and 140 prevent an optical signal from proceeding backward.
The WDM 110 couples signal light and pump light into a fiber and are transmitted in the fiber to the EDF 130. The optical isolator 100 before the WDM 110 prevents the optical signal from being amplified inefficiently by amplified spontaneous emission (ASE), generated by the EDF 130, which is reflected by an optical device, such as a signal input connector, and reenters the EDF 130. Similarly, the isolator 140 after the EDF 130 prevents the amplification efficiency of the EDFA from being degraded due to ASE which is reflected by an optical device, such as a signal output connector, and is again incident upon the EDF 130. The pump LD 120 has a wavelength of 980 nm, pumps laser forward (i.e., in the direction the signal light proceeds), and generates a fixed power of 120 mW. The EDF 130 is an EDF doped with aluminum, and the erbium concentration is 290 ppm.
Meanwhile, the optical fiber amplifier transforms pump light energy into signal light energy using the principles of a laser. Accordingly, when the intensity of the signal light is sufficiently low, the normal optical fiber amplifier cannot effectively change the erbium sufficiently excited by a pump light of high intensity into a ground state, since the signal light passes through the EDF once. Thus, the amplification efficiency of the general optical fiber amplifier is not high. That is, the energy of the pump light cannot be effectively transformed into the energy of the signal light. Therefore, there is a limit in increasing the gain of a small signal, and the difference in gain between the wavelengths of the signal light is comparatively wide.